At present, data transmission between an access point (AP) and a mobile terminal (MT) is in the form of frame during the packet transmission of a mobile communication system (e.g., OFDM system). For example, a US patent entitled Device and method for symbol frame synchronization of OFDM transmitter and receiver with the publication number of 20030193970 disclosed a frame structure. As shown in FIG. 1, the frame includes frame synchronization pattern (FSP), which is inserted into the starting portion of the frame for use of frame synchronization, and OFDM symbols following thereafter, which includes guard interval (GI) and synchronization pattern (Sync1, Sync2) or includes guard interval and data (data 1, data 2 . . . ). In a mobile communication system, every time the MT receives a frame, it must do Fast Fourier Transform (FFT) to demodulate the Time-Frequency Units Allocation Channel to check whether there is any packet to be received, which causes unnecessary power consumption and battery life shorten of mobile terminals in the system because of the high complexity of FFT.